Rechargeable lithium batteries are the result of recent advances in the field of electrochemical cells. The advantageous characteristics of such electrochemical cells include light weight, long service life, relatively high energy densities and high specific energies.
One particular group of conventional non-aqueous lithium ion batteries has a positive electrode having a metal oxide compound as the cathode active component, that readily reacts with lithium ions thus consuming electrons on discharge and releases lithium ions and electrons in the charging step. The negative electrode of this type of conventional non-aqueous lithium battery contains elemental lithium in some form, often intercalated in some kind of carbon. The lithium is ionised on discharge giving up an electron, while in the charging step lithium ions are intercalated in the carbon to form Li.sub.x C.sub.6, where 0&lt;x&lt;1, consuming electrons. The electrolyte is conveniently an organic compound containing a lithium salt that readily dissociates yielding mobile lithium ions. The conventional electrolyte of a non-aqueous lithium battery is conductive only of lithium ions and not of electrons.
It may be assumed that the greater the area of contact between the respective electrode and the appropriate portion of the electrolyte, the higher is the current density that the battery is capable of generating. There are known methods for extending the contact areas of the respective electrode-electrolyte pair in a lithium battery. U.S. Pat. No. 5,352,548, issued to Fujimoto et al. on Oct. 4, 1994, describes a lithium ion containing electrochemical cell, in which the lithium-cobalt oxide containing positive electrode is carried on an aluminum foil and the negative electrode contains a graphite-polytetrafluoro-ethylene or graphite-polyvinylidene-fluoride mixture, carried on a thin copper plate. Between the electrodes is placed a porous polypropylene film impregnated with an organic solvent containing a lithium salt. The layered electrode-electrolyte-electrode structure is rolled into a coil and packed into a cylindrical battery container having electrical leads. It is to be noted that both the negative and the positive electrode mixtures require a binder to keep the particular electrode layer structurally stable, coherent and conductive. The smaller radii and tightly curving surface of the portions close to the central axis of the coil are likely to put substantial mechanical stress on the electrode layers resulting in reduced efficiency of such concentrically coiled electrochemical cells.
The non-aqueous lithium- battery described in U.S. Pat. No. 4,830,940, issued to Keister et al. on May 16, 1989, has a continuous, folded elemental lithium metal bearing, substantially rigid anode (negative electrode), and pellets of silver-vanadium oxide containing a binder, serving as cathode (positive electrode), sealed in a separator envelope located between the folds. The anode of U.S. Pat. No. 4,830,940 is constructed of a rigidly folded metallic mesh coated with a lithium foil on each of its faces, and enclosed in a polypropylene or polyethylene separator. The rigidly folded anode structure having pellets of cathodes between its folds, is immersed in an organic liquid electrolyte containing a lithium salt. It is noted that both the negative (anode) and the positive (cathode) active materials are enclosed in separator sheets and are not in direct contact with the electrolyte.
U.S. Pat. No. 5,300,373, issued to D. R. Shackle on Apr. 5, 1994, describes fan-folded, stacked electrochemical lithium cells. The battery of Shackle is made of an elongated laminate of one polarity electrode in continuous contact with a solid polymer electrolyte laminate. Both the electrode and the electrolyte laminates in continuous contact, are fan-folded and segments of the second polarity electrode are sandwiched between the folds. The various embodiments shown in U.S. Pat. No. 5,300,373 differ only in the manner the generated current is collected from the second polarity electrodes, however, all embodiments show folded double layers of the first polarity electrode and the electrolyte, the pair having similar widths. The first polarity electrode is carried as a continuous layer on a conductive web, hence it may be assumed that it contains a binder and has a definite thickness. The solid electrolyte laminate is also shown to have a definite thickness. One of the often experienced problems with fan-folding such double layered structures is that the pressure exerted on the inner folded layer at the highly curved corners results in what is known as "dog-bone" shaped distortions, causing the outer layer to be stretched over the "dog-bone" end. Such distortions lead to cracks, breaks, crumbling and similar manifestations of mechanical stress, all of which are likely to reduce the efficiency of performance and service life of the stacked battery. Thus lithium batteries made of conventional stacked electrochemical cells usually have weak areas at the high curvature bends of the folds. Furthermore, in some of the embodiments described in U.S. Pat. No. 5,300,373 issued to Shackle, the second electrode segments are only in edge contact with the current collector, hence a small movement during packing or use of the stacked cells may result in rendering some of the second polarity electrode segments malfunctional. Another difficulty which may arise due to the continuous first polarity electrode having the same width as the solid polymer electrolyte, is that the second polarity electrode or its current collector may come into direct contact with the first polarity electrode, thereby short circuiting the folded cell.
There is a need for an improved rechargeable non-aqueous lithium battery made of stacked folded electrochemical cells, that overcomes the above problems.